High strength steel plate having low yield ratio excellent in terms of strain ageing resistance, method of manufacturing the same and high strength welded steel pipe made of the same

ABSTRACT

A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure including a bainite phase and island martensite, and a polygonal ferrite in surface portions within 5 mm from the upper and lower surfaces, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 μm or less, the area fraction of the polygonal ferrite in the surface portions is 10% to less than 80%.

TECHNICAL FIELD

This disclosure relates to a steel plate having a low yield ratio, high strength and high toughness preferably used mainly in a linepipe field, a method of manufacturing the steel plate and a high strength welded steel pipe, in particular, to a steel plate having a low yield ratio, high strength and high toughness excellent in terms of strain ageing resistance, a method of manufacturing the steel plate and a high strength welded steel pipe excellent in terms of buckling resistance and ductility which is made of the steel plate.

BACKGROUND

Nowadays, steel materials for welded structures are required to have not only high strength and high toughness, but also a low yield ratio and high uniform elongation from the viewpoint of earthquake resistance. Generally, it is known that, by forming a metallographic structure of the steel material in which hard phases such as a bainite phase (hereinafter, also referred to as β) and a martensite phase (hereinafter, also referred to as M) are appropriately dispersed in a ferrite phase (hereinafter, referred to as a) which is a soft phase, it is possible to achieve a decrease in the yield ratio of a steel material and an increase in the uniform elongation of a steel material.

As an example of manufacturing methods of forming a microstructure in which hard phases are appropriately dispersed in a soft phase as described above, a certain method is described in Japanese Unexamined Patent Application Publication No. 55-97425. That is, JP '425 discloses a heat treatment method in which, as an intermediate treatment between quenching (hereinafter, also referred to as Q) and tempering (hereinafter, also referred to as T), quenching starting from a temperature range for forming a dual phase consisting of a ferrite phase and an austenite phase (hereinafter, also referred to as γ) is performed (hereinafter, also referred to as Q′).

Japanese Unexamined Patent Application Publication No. 55-41927 discloses an example of a method which does not require additional manufacturing processes, in which, after rolling has been finished at a temperature equal to or higher than the Ar₃ transformation point, the start of accelerated cooling is delayed until the steel material has a temperature equal to or lower than the Ar₃ transformation point at which a ferrite phase is formed.

As an example of a technique with which it is possible to achieve a decrease in yield ratio without performing complicated heat treatments as disclosed in JP '425 and JP '927, Japanese Unexamined Patent Application Publication No. 1-176027 discloses a method with which a decrease in yield ratio is achieved by finishing rolling of a steel material at a temperature equal to or higher than the Ar₃ transformation point, and then by controlling an accelerated cooling rate and a cooling stop temperature to form a dual-phase structure consisting of an acicular ferrite phase and a martensite phase.

Moreover, Japanese Patent No. 4066905 discloses an example of a technique with which it is possible to achieve a low yield ratio and excellent toughness in a weld heat affected zone without significantly increasing the contents of alloying chemical elements of a steel material, in which, by controlling Ti/N and a Ca—O—S balance, a three-phase structure consisting of a ferrite phase, a bainite phase and island martensite (hereinafter, also referred to as MA) is formed.

In addition, Japanese Unexamined Patent Application Publication No. 2008-248328 discloses a technique in which a decrease in yield ratio and an increase in uniform elongation are achieved by adding alloying chemical elements such as Cu, Ni and Mo.

On the other hand, in a welded steel pipe used to form a linepipe such as a UOE steel pipe or an electric resistance welded steel pipe, after a steel plate has been formed by cold forming into a cylindrical shape and the butted portions have been welded, usually, a coating treatment such as polyethylene coating or powder epoxy coating is performed on the outer surface of the resultant steel pipe from the viewpoint of, for example, corrosion protection. Therefore, there is a problem in that, since strain ageing occurs due to working strain applied when pipe forming is performed and due to heating when the coating treatment is performed, there is an increase in yield stress, which results in the yield ratio of the steel pipe being larger than that of the steel plate.

To solve the problem described above, for example, Japanese Unexamined Patent Application Publication Nos. 2005-60839 and 2005-60840 disclose steel pipes having a low yield ratio, high strength and high toughness excellent in terms of strain ageing resistance and methods of manufacturing the steel pipes utilizing the fine precipitations of complex carbides containing Ti and Mo or the fine precipitations of complex carbides containing two or all of Ti, Nb, and V.

In the heat treatment method according to JP '425, by appropriately selecting a quenching temperature in a range forming a dual phase, it is possible to achieve a decrease in yield ratio, but there is a problem in that there is a decrease in productivity and there is an increase in manufacturing cost due to an increase in the number of heat treatment processes.

In addition, in the technique according to JP '927, there is a problem in that there is a significant decrease in productivity, since it is necessary to perform cooling at a cooling rate equivalent to a spontaneous cooling rate in a temperature range from a rolling finish temperature to an accelerated cooling start temperature.

Moreover, in the technique according to JP '027, as indicated by the examples in the literature, there is a problem in that, since the carbon content or the contents of other alloying chemical elements of a steel plate are increased to obtain a steel material having a tensile strength of 490 N/mm² (50 kg/mm²) or more, there is an increase in material cost and, in addition to that, there is a decrease in toughness in a weld heat affected zone (HAZ).

In addition, in the technique according to JP '905, the influence of, for example, a microstructure on uniform elongation which a steel plate is required to have when the steel plate is used for, for example, a linepipe has not necessarily been clarified. In addition, since the low-temperature toughness of a base metal was evaluated only at a temperature of −10° C., it is not clear whether or not the steel plate can be used for a new use application in which toughness at a lower temperature is required.

In the technique according to JP '328, there is a problem in that there is an increase in material cost, and, in addition to that, there is a decrease in toughness in a weld heat affected zone, since it is necessary that the steel plate has a chemical composition containing increased contents of alloying chemical elements. In addition, the low-temperature toughness of a base metal and a weld heat affected zone are evaluated only at a temperature of −10° C.

In the techniques according to JP '839 and JP '840, there is an increase in strain ageing resistance, however, the low-temperature toughness of a base metal and a weld heat affected zone are evaluated only at a temperature of −10° C.

Moreover, in JP '425, JP '927, JP '027, JP '905, JP '328, JP '839 and JP '840, it is necessary to form a ferrite phase, but the ferrite phase causes a decrease in tensile strength. Therefore, it is necessary to add alloying chemical elements to increase strength to an X60 grade or more according to the API standard, and there is concern that there may be an increase in alloy cost and there may be a decrease in low-temperature toughness.

Therefore, it could be helpful to provide a steel plate having a low yield ratio, high strength and high toughness excellent in terms of strain ageing resistance of an API 5L X70 grade or less which can be manufactured at high productivity, a method of manufacturing the steel plate, and a high strength welded steel pipe made of the steel plate.

SUMMARY

We thus provide:

-   -   [1] A high strength steel plate having a low yield ratio, the         steel plate having a chemical composition containing, by mass %,         C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0%         or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less,         S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and         0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010%         or less, O: 0.005% or less and the balance being Fe and         inevitable impurities, a metallographic structure including a         bainite phase and island martensite, and further including a         polygonal ferrite in surface portions within 5 mm from the upper         and lower surfaces, in which the area fraction of the island         martensite is 3% or more and 15% or less, in which the         equivalent circle diameter of the island martensite is 3.0 μm or         less, in which the area fraction of the polygonal ferrite in the         surface portions is 10% or more and less than 80%, and in which         the remainder of the metallographic structure is a bainite         phase, a hardness variation in the thickness direction of ΔHV30         or less in terms of Vickers hardness, a hardness variation in         the width direction of ΔHV30 or less in terms of Vickers         hardness, a maximum hardness in the surface portions of the         steel plate of HV230 or less in terms of Vickers hardness and a         yield ratio of 85% or less and an elongation of 22% or more in a         full-thickness tensile test using a test piece having a shape in         accordance with GOST standards.     -   [2] The high strength steel plate having a low yield ratio         according to item [1], the steel plate having the chemical         composition further containing, by mass %, one or more selected         from among Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less,         Mo: 0.5% or less, V: 0.1% or less, Ca: 0.0005% or more and         0.003% or less and B: 0.005% or less.     -   [3] A method of manufacturing a high strength steel plate having         a low yield ratio, the method including heating steel having the         chemical composition according to item [1] or [2] at a         temperature of 1000° C. or higher and 1300° C. or lower,         performing hot rolling under conditions such that the cumulative         rolling reduction ratio is 50% or more in a temperature range of         900° C. or lower, finishing hot rolling at a temperature equal         to or higher than the Ar₃ transformation point, starting cooling         when the temperature of the surface portions within 5 mm from         the upper and lower surfaces is equal to or higher than (the Ar₃         transformation temperature −60° C.) and equal to or lower than         the Ar₃ transformation point, performing cooling at a cooling         rate of 200° C./sec. or less in terms of the surface temperature         of the steel plate until the surface temperature becomes 600° C.         or lower, performing cooling at a cooling rate of 15° C./sec. or         more in terms of the average temperature of the steel plate         until the average temperature becomes 450° C. or higher and         650° C. or lower, and, immediately after the cooling has been         performed, performing reheating at a heating rate of 1.0°         C./sec. or more in terms of the surface temperature of the steel         plate until the surface temperature becomes 550° C. or higher         and 750° C. or lower.     -   [4] A high strength welded steel pipe, the steel pipe being         manufactured by forming the steel plate according to item [1] or         [2] into a cylindrical shape, by welding butted portions of the         shaped cylinder under conditions such that single-layer welding         is performed on each of the outer and inner surfaces, and by         thereafter performing a pipe-expanding treatment under a         condition of a pipe expanding ratio of 0.4% or more and 2.0% or         less, and having a yield ratio of 90% or less and an elongation         of 20% or more in a full-thickness tensile test using a test         piece having a shape in accordance with GOST standards, and,         further having a yield ratio of 90% or less and an elongation of         20% or more even after a strain ageing treatment has been         performed under conditions such that the temperature is 250° C.         or lower and the duration is 30 minutes or less.

A steel plate having a low yield ratio, high strength and high toughness excellent in terms of strain ageing resistance can be manufactured without decreasing toughness in a weld heat affected zone or adding a large amount of alloying chemical elements. Therefore, a steel plate used mainly for a linepipe can be stably manufactured in a large amount, and there is a significant increase in productivity and economic efficiency. Moreover, since a high strength welded steel pipe excellent in terms of buckling resistance and ductility can be manufactured using this steel plate, there is a significant industrial effect.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram illustrating a thermal history to which a steel plate is subjected after accelerated cooling has been started, in which a solid line represents the average temperature of the steel plate and a dot-dash line represents the surface temperature of the steel plate.

DETAILED DESCRIPTION

We conducted investigations regarding a method of manufacturing a steel plate, in particular, regarding a manufacturing process including controlled rolling, accelerated cooling after controlled rolling has been performed and subsequent reheating. We found that, by forming a polygonal ferrite phase only in the surface portion of a steel plate and by controlling rolling conditions so that a bainite phase in the surface portion of the steel plate becomes soft, it is possible to achieve high deformation performance and high ductility without a significant decrease in strength, and obtained the following findings:

-   -   (a) By stopping accelerated cooling in the middle of an         accelerated cooling process in which a steel plate undergoes         bainite transformation, that is, in a temperature range in which         a non-transformed austenite phase is present, and by         subsequently starting reheating from a temperature higher than a         temperature at which bainite transformation finishes         (hereinafter, referred to as a Bf point), a microstructure of         the metallographic structure of the steel plate, in which hard         island martensite is uniformly formed in a bainite phase, is         formed, and thus there is a decrease in yield ratio.     -   (b) By adding an appropriate amount of Mn to the chemical         composition of steel as a chemical element for stabilizing an         austenite phase, a non-transformed austenite phase become         stabilized and, thus, it is possible to form hard MA without         adding a large amount of chemical elements for increasing         hardenability such as Cu, Ni, and Mo.     -   (c) By performing rolling under conditions such that the         cumulative rolling reduction ratio is 50% or more in a         temperature range of 900° C. or lower, which is a         no-recrystallization temperature range in austenite, fine MA can         be uniformly dispersed, and thus there is an increase in uniform         elongation while a low yield ratio is maintained.     -   (d) By appropriately controlling both the rolling conditions in         the no-recrystallization temperature range in austenite         described in item (c) above and the reheating conditions         described in item (a) above, the shape of MA can be controlled.         That is, the grain size of MA can be controlled to be as small         as 3.0 μm or less in terms of average equivalent circle         diameter. As a result, since MA is only slightly decomposed even         if steel is subjected to a thermal history which causes         deterioration of a yield ratio due to ageing in the case of a         conventional steel, the desired microstructure and properties         can be maintained even after ageing has occurred.     -   (e) By starting the cooling of a steel plate when the         temperature of surface portions within 5 mm from the upper and         lower surfaces of the steel plate is equal to or higher than         (the Ar₃ transformation temperature −60° C.) and equal to or         lower than the Ar₃ transformation temperature, a polygonal         ferrite phase of an appropriate area ratio can be formed in the         surface portions within 5 mm from the upper and lower surfaces.         As a result, since there is a decrease in the hardness of the         surface portions described above, high ductility can be         achieved.     -   (f) By performing a first stage cooling at a cooling rate of         200° C./sec. or less until the temperature of a steel plate         becomes 600° C. or lower, a bainite structure in the surface         portions can be softened. As a result, since there is a decrease         in hardness of the surface portions of a steel plate, high         ductility can be achieved.

The reason for limitations on the features will be described hereafter.

1. Regarding Chemical Composition

First, the reason for the limitations on the chemical composition of the steel will be described. % used when describing a chemical composition always represents mass %.

C: 0.03% or more and 0.08% or less

C is a chemical element which contributes by forming carbide to precipitation strengthening and is important to form MA. When the C content is less than 0.03%, the content is not sufficient to form MA and sufficient strength cannot be achieved. When the C content is more than 0.08%, there is a decrease in the toughness of a base metal and toughness in a weld heat affected zone (HAZ). Therefore, the C content is 0.03% or more and 0.08% or less, preferably 0.04% or more and 0.06% or less.

Si: 0.01% or more and 1.0% or less

Si is added to perform deoxidation. When the Si content is less than 0.01%, there is an insufficient effect of deoxidation and, when the Si content is more than 1.0%, there is a decrease in toughness and weldability. Therefore, the Si content is 0.01% or more and 1.0% or less, preferably 0.01% or more and 0.3% or less.

Mn: 1.2% or more and 3.0% or less

Mn is added to increase strength and toughness. Mn is also added to increase hardenability that promotes formation of MA. When the Mn content is less than 1.2%, these effects cannot be sufficiently obtained and, when the Mn content is more than 3.0%, there is a decrease in toughness and weldability. Therefore, the Mn content is 1.2% or more and 3.0% or less, preferably 1.8% or more and 3.0% or less to stably form MA regardless of variations in a chemical composition and manufacturing conditions.

P: 0.015% or less

P is an inevitable impurity, and the upper limit of the P content is specified. When the P content is large, there is a significant increase in the degree of central segregation, resulting in a decrease in the toughness of a base metal. Therefore, the P content is 0.015% or less, preferably 0.010% or less.

S: 0.005% or less

S is an inevitable impurity, and the upper limit of the S content is specified. When the S content is large, there is a significant increase in the amount of MnS formed, resulting in a decrease in the toughness of a base metal. Therefore, the S content is 0.005% or less, preferably 0.002% or less.

Al: 0.08% or less

Al is added as a deoxidation agent. When the Al content is more than 0.08%, there is a decrease in the cleanliness of steel, resulting in a decrease in toughness. Therefore, the Al content is 0.08% or less, preferably 0.01% or more and 0.08% or less, more preferably 0.01% or more and 0.05% or less.

Nb: 0.005% or more and 0.07% or less

Nb is a chemical element which increases toughness as a result of decreasing the grain size in the microstructure and contributes to an increase in strength due to an increase in hardenability through the use of solute Nb. These effects are obtained when the Nb content is 0.005% or more. However, when the Nb content is more than 0.07%, there is a decrease in toughness in a weld heat affected zone, and thus the Nb content is 0.005% or more and 0.07% or less, more preferably 0.01% or more and 0.05% or less.

Ti: 0.005% or more and 0.025% or less

Ti is an important chemical element which increases the toughness of a base metal as a result of preventing an increase in the grain size of an austenite phase through the use of the pinning effect of TiN when slab heating is performed. This effect is obtained when the Ti content is 0.005% or more. However, when the Ti content is more than 0.025%, there is a decrease in toughness in a weld heat affected zone, and thus the Ti content is 0.005% or more and 0.025% or less, preferably 0.005% or more and less than 0.02% from the view point of toughness in a weld heat affected zone, more preferably 0.007% or more and 0.016% or less.

N: 0.010% or less

N is treated as an inevitable impurity. Since there is a decrease in toughness in a weld heat affected zone when the N content is more than 0.010%, the N content is 0.010% or less, preferably 0.007% or less, more preferably 0.006% or less.

O: 0.005% or less

O is an inevitable impurity and the upper limit of the O content is specified. Since O causes formation of coarse inclusions which has a negative effect on toughness, the O content is 0.005% or less, more preferably 0.003% or less.

The basic chemical composition is as described above. Furthermore, to improve the strength and toughness of a steel plate and to increase hardenability to promote formation of MA, one or more of Cu, Ni, Cr, Mo, V, Ca, and B described below may be added.

Cu: 0.5% or less

Since Cu contributes to an increase in the hardenability of steel when Cu is added, Cu may be added. It is preferable that the Cu content be 0.05% or more to obtain this effect. However, when the Cu content is 0.5% or more, there is a decrease in toughness. Therefore, when Cu is added, it is preferable that the Cu content be 0.5% or less, more preferably 0.4% or less.

Ni: 1% or less

Since Ni contributes to an increase in the hardenability of steel and, in particular, since there is not a decrease in toughness even when the Ni content is large, Ni is effective to increase toughness. Therefore, Ni may be added. It is preferable that the Ni content be 0.05% or more to obtain this effect. However, since Ni is an expensive chemical element, when Ni is added, it is preferable that the Ni content be 1% or less, more preferably 0.4% or less.

Cr: 0.5% or less

Since Cr is, like Mn, a chemical element effective to achieve sufficient strength even when the C content is low, Cr may be added. Although it is preferable that the Cr content be 0.1% or more to obtain this effect, there is a decrease in weldability when the Cr content is excessively large. Therefore, when Cr is added, it is preferable that the Cr content be 0.5% or less, more preferably 0.4% or less.

Mo: 0.5% or less

Since Mo is a chemical element which increases hardenability, and since Mo is a chemical element which contributes to an increase in strength as a result of formation of MA and strengthening a bainite phase, Mo may be added. It is preferable that the Mo content be 0.05% or more to obtain this effect. However, when the Mo content is more than 0.5%, there is a decrease in toughness in a weld heat affected zone. Therefore, when Mo is added, it is preferable that the Mo content be 0.5% or less, more preferably 0.3% or less.

V: 0.1% or less

Since V is a chemical element which contributes to an increase in strength as a result of increasing hardenability, V may be added. It is preferable that the V content be 0.005% or more to obtain this effect. However, when the V content is more than 0.1%, there is a decrease in toughness in a weld heat affected zone. Therefore, when V is added, it is preferable that the V content be 0.1% or less, more preferably 0.06% or less.

Ca: 0.0005% or more and 0.003% or less

Since Ca is a chemical element which increases toughness as a result of controlling the shape of inclusions of sulfide, Ca may be added. This effect is realized when the Ca content is 0.0005% or more. When the Ca content is more than 0.003%, this effect becomes saturated and there is conversely a decrease in toughness as a result of decreasing cleanliness. Therefore, when Ca is added, it is preferable that the Ca content be 0.0005% or more and 0.003% or less, more preferably 0.001% or more and 0.003% or less.

B: 0.005% or less

Since B is a chemical element which contributes to an increase in strength and an increase in toughness in a weld heat affected zone, B may be added. It is preferable that the B content be 0.0005% or more to obtain these effects. However, when the B content exceeds 0.005%, there is a decrease in weldability. Therefore, when B is added, it is preferable that the B content be 0.005% or less, more preferably 0.003% or less.

By controlling the ratio of the Ti content and the N content Ti/N, it is possible to prevent an increase in the grain size of an austenite phase in a weld heat affected zone as a result of presence of TiN particles, and it is possible to achieve good toughness in a weld heat affected zone. Therefore, it is preferable that Ti/N be 2 or more and 8 or less, more preferably 2 or more and 5 or less.

The remainder of the chemical composition other than those described above in a steel plate consists of Fe and inevitable impurities. However, as long as the desired effects are not decreased, chemical elements other than those described above may be added. For example, from the viewpoint of increasing toughness, Mg: 0.02% or less and/or REM (rare-earth metal): 0.02% or less may be added.

Subsequently, the metallographic structure will be described.

2. Regarding Metallographic Structure

The metallographic structure is controlled so that a duplex-phase structure consisting of a bainite phase and island martensite is a main body of the metallographic microstructure and areas (surface portions) within 5 mm from the upper and lower surfaces have a three-phase structure consisting of a polygonal ferrite, a bainite phase, and island martensite.

By forming a duplex-phase structure serving as a main body of the structure, in which hard MA is included in a bainite phase, a decrease in yield ratio, an increase in uniform elongation, and an increase in strength are obtained. In addition, by forming a three-phase structure including a polygonal ferrite phase in surface portions within 5 mm from the upper and lower surfaces, an increase in uniform elongation and an increase in ductility are obtained.

When steel plates and steel pipes are used in regions such as earthquake regions in which a large deformation is applied to these materials, these materials may be required to have not only a low yield ratio but also high uniform elongation and high ductility. In multi-phase structures consisting of a soft polygonal ferrite, a bainite phase and hard MA as described above, the soft phase undergoes deformation. In addition, by controlling hardness variations in the thickness direction and in the width direction to be about ΔHV30 or less and a maximum hardness in the surface portions of the steel plate to be about HV230 or less, it is possible to achieve a high elongation of 22% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards for a steel plate.

The content ratio of MA in a metallographic structure is 3% or more and 15% or less in terms of area fraction of MA (calculated as the average value of the area ratios of MA in arbitrary cross sections in the rolling direction, the width direction and the like of a steel plate). When the area fraction of MA is less than 3%, there may an insufficient decrease in yield ratio. When the area fraction of MA is more than 15%, there may be a decrease in the toughness of a base metal.

In addition, it is preferable that the area fraction of MA be 5% or more and 15% or less from the viewpoint of decreasing yield ratio and increasing uniform elongation and base-material toughness. The area fraction of MA means the ratio with respect to the whole microstructure of steel.

MA can be easily identified by observing a sample prepared by etching a steel plate using, for example, a 3% nital solution (nital: nitric acid alcohol solution) and by subsequently performing electrolytic etching. By observing the microstructure of a steel plate using a scanning electron microscope (SEM), MA is recognized as a distinct white portion.

The area fraction of MA can be calculated, for example, as the average value of the area ratios constituted by MA by performing image processing on microstructure photographs of at least 4 microscopic fields taken through observation using a scanning electron microscope (SEM).

In addition, the equivalent circle diameter of MA is 3.0 μm or less from the viewpoint of achieving sufficient toughness for a base metal and increasing uniform elongation. This is because, when the equivalent circle diameter of MA is more than 3.0 μm, there may be a decrease in the toughness of a base metal.

The equivalent circle diameter of MA can be calculated as the average value of the diameters of the circles respectively having the same areas as MA grains obtained by performing image processing on microstructure taken through observation using a SEM.

Mn and Si are added to form MA without adding large amounts of expensive alloying chemical elements such as Cu, Ni, and Mo. It is important to stabilize a non-transformed austenite phase with this method to suppress pearlite transformation or formation of a cementite phase when air-cooling is performed after reheating has been performed.

The mechanism to form MA and a polygonal ferrite phase in the upper and lower surface portions will be roughly described hereafter. Manufacturing conditions will be described in detail later.

After having heated a slab, hot rolling is finished in a temperature range for forming an austenite phase, and then accelerated cooling is started at a temperature just below the Ar₃ transformation temperature.

In a manufacturing process, in which, after accelerated cooling has been finished in the middle of a bainite transformation process, that is, in a temperature range in which a non-transformed austenite phase is present, reheating is started at a temperature higher than a temperature at which bainite transformation (Bf point) is finished, and then cooling is performed, changes in a microstructure will be described hereafter.

When accelerated cooling is finished, the microstructure consists of a polygonal ferrite phase in the upper and lower surface portions, a bainite phase, and a non-transformed austenite phase. After that, by performing reheating starting at a temperature higher than the Bf point, transformation from a non-transformed austenite phase to a bainite phase occurs and, since the solid solubility limit of C in a bainite phase is small, C is evacuated into the surrounding non-transformed austenite phase.

Therefore, the C content in a non-transformed austenite phase increases as bainite transformation progresses when reheating is performed. At this time, when austenite stabilizing chemical elements such as Cu and Ni are added in certain amounts or more, a non-transformed austenite phase in which C is concentrated are retained even after reheating has been finished and transforms into MA when cooling is performed after reheating has been performed and, fin-ally, a microstructure in which MA is formed in a bainite structure is formed. The microstructure further including a polygonal ferrite phase is formed in the upper and lower surface portions.

The area fraction of a polygonal ferrite phase in the surface portions is 10% or more and less than 80%. This is because, when the area fraction of an polygonal ferrite phase in the surface portions is less than 10%, the surface of a steel plate has an excessively high hardness of more than HV230, resulting in a case where elongation is less than 22%. In addition, this is because, when the area fraction of a polygonal ferrite phase in the surface portions is 80% or more, there is an excessive decrease in strength of a steel plate.

It is important to start reheating in a temperature range in which a non-transformed austenite phase is present after accelerated cooling has been performed. When the reheating-start temperature is equal to or lower than the Bf point, bainite transformation is completed, and a non-transformed austenite phase is not present. Therefore, it is necessary that the reheating-start temperature be higher than the Bf point.

In addition, there is no particular limitation on what cooling method is used after reheating has been performed because it has no influence on MA transformation. However, it is basically preferable that air-cooling be used. By using steel containing a certain amount of Mn, by stopping accelerated cooling in the middle of a bainite transforming process and by subsequently starting reheating immediately after the cooling has been stopped, hard MA can be formed without decreasing productivity.

Although the steel plate has a metallographic structure in which a certain amount of MA is uniformly included in a bainite phase and in which a polygonal ferrite phase is further included in the surface portions within 5 mm from the upper and lower surfaces, this disclosure includes a steel plate containing other types of microstructures and precipitations as long as the desired effects are not decreased.

Specifically, when one, two or more of other types of microstructures such as a pearlite phase and a cementite phase are formed together in addition, there is a decrease in strength. However, when the area fractions of the microstructures other than a polygonal ferrite phase, a bainite phase and MA is small, an effect of decreasing strength is negligible. Therefore, as long as the total area fraction of a metallographic structures other than the three kinds of microstructures, which are a polygonal ferrite phase, a bainite phase and MA, is 3% or less with respect to the whole microstructure, one or more of the metallographic structures such as a pearlite phase, a cementite phase and the like may be included.

The metallographic structure described above can be formed using steel having the chemical composition described above and the manufacturing methods described hereafter.

3. Regarding Manufacturing Conditions

It is preferable that steel having the chemical composition described above be refined with a common method using a refining means such as a converter furnace or an electric furnace and be made into a steel material such as a slab using a common method such as a continuous casting method or an ingot casting-slabbing method. The refining method and a casting method are not limited to those described above. After that, the slab is rolled into a desired shape, and cooling and heating are performed after rolling has been performed.

The cooling start temperature is expressed in terms of the surface temperature of a steel plate, and a cooling rate and a cooling stop temperature are expressed in terms of both the surface temperature of a steel plate and the average temperature of a steel plate, unless otherwise noted. Other temperatures such as a slab heating temperature, a controlled rolling start temperature, a controlled rolling finish temperature and a reheating temperature in a reheating process are expressed in terms of the average temperature of a steel plate.

The average temperature of a steel plate is calculated from the surface temperature of a slab or a steel plate in consideration of parameters such as thickness and thermal conductivity. In addition, the cooling rate is an average cooling rate derived by dividing a temperature difference necessary in a cooling process after hot rolling has been finished down to a cooling stop temperature (450° C. to 650° C.) by a time spent for the cooling.

In addition, the heating rate is an average heating rate derived by dividing a temperature difference necessary in a reheating process after cooling has been finished up to a reheating temperature (550° C. to 750° C.) by a time spent for the reheating. Manufacturing conditions will be described in detail hereafter.

As the Ar₃ temperature, the value derived using the following equation will be used: Ar₃(° C.)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo, where an atomic symbol represents the mass % of the chemical element represented by the symbol. Heating temperature: 1000° C. or higher and 1300° C. or lower

When the heating temperature is lower than 1000° C., a sufficient solution of carbides cannot be achieved, and the desired strength cannot be achieved. In the case where the heating temperature is higher than 1300° C., there is a decrease in the toughness of a base metal. Therefore, the heating temperature is 1000° C. or higher and 1300° C. or lower.

Rolling finish temperature: equal to or higher than the Ar₃ temperature

When the rolling finish temperature is lower than the Ar₃ temperature, there is a decrease in ferrite transformation speed after rolling has been finished, and there is insufficient concentration of C in a non-transformed austenite phase when reheating is performed, which results in MA not being formed. Therefore, the rolling finish temperature is equal to or higher than the Ar₃ temperature.

Cumulative rolling reduction ratio in a temperature range of 900° C. or lower: 50% or more

A temperature range of 900° C. or lower corresponds to the lower part of a no-recrystallization temperature range in austenite. By controlling the cumulative rolling reduction ratio to be 50% or more in this temperature range, a decrease in austenite grain size can be achieved. With this method, there is an increase in the number of MA forming sites at prior-austenite grain boundaries afterward, which contributes to the suppression of an increase in MA grain size.

When the cumulative rolling reduction ratio in a temperature range of 900° C. or lower is less than 50%, the equivalent circle diameter of formed MA becomes more than 3.0 μm, resulting in a case where there is a decrease in uniform elongation and/or there is a decrease in the toughness of a base metal. Therefore, the cumulative rolling reduction ratio in a temperature range of 900° C. or lower is 50% or more.

FIG. 1 is a schematic diagram illustrating a cooling curve expressed in terms of the average temperature of a steel plate and a cooling-heating curve expressed in terms of the surface temperature of a steel plate in an accelerated cooling process.

A cooling start temperature is equal to or higher than (the Ar₃ transformation temperature −60° C.) and equal to or lower than the Ar₃ transformation temperature in terms of the surface temperature of a steel plate. This condition is one of the important manufacturing conditions. By controlling the accelerated cooling start temperature after rolling has been finished to be equal to or lower than the Ar₃ transformation temperature and equal to or higher than (the Ar₃ transformation temperature −60° C.) in terms of the surface temperature of a steel plate, a polygonal ferrite can be formed in an amount of 10% or more and less than 80% in terms of area fraction in surface portions within 5 mm from the upper and lower surfaces of the steel plate. As a result, the maximum hardness of the surface portions of the steel plate can be controlled to be HV230 or less. Thus, the elongation of the steel plate can be controlled to be 22% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards.

Regarding the cooling conditions of the steel plate, the cooling rate is 200° C./sec. or less and the cooling stop temperature is 600° C. or lower in terms of the surface temperature of the steel plate.

By performing cooling under conditions such that the cooling rate is 200° C./sec. or less and the cooling stop temperature is 600° C. or lower, a bainite structure in the surface portions of the steel plate becomes soft, and thus the maximum hardness of the surface portions of the steel plate can be controlled to be HV230 or less in terms of Vickers hardness, and hardness variations in the thickness direction and in the width direction can be suppressed to be ΔHV30 or less. By controlling hardness variations in the thickness and width directions to be ΔHV30 or less and by controlling the maximum hardness of the surface portions to be HV230 or less as described above, the elongation variation and a decrease in elongation of the steel plate can be suppressed, and an elongation of 22% or more can be stably achieved.

In addition, the reason why the yield ratio of a steel plate is 85% or less and the elongation of the steel plate is 22% or more will be described hereafter. This is because it is necessary that the target value of the yield ratio be 85% or less and the target value of the elongation be 22% or more at the stage of the steel plate to achieve a yield ratio of 90% or less and an elongation of 20% or more at the stage of a steel pipe in consideration of changes in the material properties due to the working strain induced by forming a steel plate to a steel pipe.

The cooling stop temperature is 600° C. or lower in terms of the surface temperature of the steel plate to control the surface temperature of the steel plate to be lower than or equal to a temperature at which bainite transformation starts, and it is preferable that the cooling stop temperature be 200° C. or higher and 500° C. or lower in terms of the surface temperature of the steel plate. With this method, the metallographic structure of surface portions within 5 mm from the upper and lower surfaces of the steel plate becomes a three-phase structure consisting of a polygonal ferrite phase, a bainite phase and MA. Incidentally, it is preferable that the lower limit of the cooling rate be 50° C./sec.

A cooling rate in terms of the average temperature of the steel plate is 15° C./sec. or more.

When the cooling rate is less than 15° C./sec., a pearlite phase is formed when cooling is performed, and sufficient strength or sufficiently low yield ratio cannot be achieved. Therefore, the cooling rate in terms of the average temperature of the steel plate is 15° C./sec. or more.

The steel plate is supercooled to a temperature range for bainite transformation by performing accelerated cooling, and it is possible to complete bainite transformation when reheating is subsequently performed without holding the steel plate at the reheating temperature.

A cooling stop temperature in terms of the average temperature of the steel plate is 450° C. or higher and 650° C. or lower.

This process is an important manufacturing condition. A non-transformed austenite phase present after reheating has been performed and in which C is concentrated, transforms into MA when air-cooling is performed after reheating has been performed.

That is, it is necessary that cooling be stopped in the middle of bainite transformation, that is, in a temperature range in which a non-transformed austenite phase is present. When the cooling stop temperature is lower than 450° C., bainite transformation is completed, and MA is not formed when air-cooling is performed and a decrease in yield ratio cannot be achieved. When the cooling stop temperature is higher than 650° C., C is consumed by a pearlite phase which is precipitated when cooling is performed, and MA is not formed. Therefore, the accelerated cooling stop temperature is 450° C. or higher and 650° C. or lower. The accelerated cooling stop temperature preferably be 500° C. or higher and 600° C. or lower from the viewpoint of achieving the area fraction of MA which is ideal for achieving better strength and toughness. Regarding this accelerated cooling, an arbitrary cooling apparatus can be used.

Immediately after accelerated cooling has been stopped, reheating is performed up to a temperature of 550° C. or higher and 750° C. or lower at a heating rate of 1.0° C./sec. or more in terms of the surface temperature of the steel plate.

“Reheating is performed immediately after accelerated cooling has been stopped” means that reheating is performed at a heating rate of 1.0° C./sec. or more within 120 seconds after accelerated cooling has been stopped.

This process is also an important manufacturing condition. While a non-transformed austenite phase transforms into a bainite phase when reheating is performed after accelerated cooling has been performed as described above, C is evacuated into a non-transformed austenite phase which is remaining. Then, the non-transformed austenite phase in which C is concentrated is transformed into MA when air-cooling is performed after reheating has been performed.

To form MA, it is necessary that reheating be performed from a temperature higher than the Bf point to a temperature of 550° C. or higher and 750° C. or lower after accelerated cooling has been performed.

When the heating rate is less than 1.0° C./sec., it takes a long time for a steel plate to reach the target reheating temperature, which results in a decrease in productivity and, there may be an increase in MA grain size. As a result, it is impossible to achieve a sufficiently low yield ratio, sufficient toughness or sufficient uniform elongation. Although the mechanisms are not necessarily clear, the reason is thought to be as follows. That is, by increasing the heating rate for reheating to be 1.0° C./sec. or more, an increase in grain size in a region in which C is concentrated is suppressed, and an increase in MA grain size which is formed in a cooling process after reheating has been performed is suppressed.

When the reheating temperature is lower than 550° C., transformation does not sufficiently progress, and a sufficient amount of C is not evacuated into a non-transformed austenite phase and a decrease in yield ratio cannot be achieved as a result of a sufficient amount of MA not being formed. When the reheating temperature is higher than 750° C., sufficient strength cannot be achieved due to the softening of a bainite phase. Therefore, the reheating temperature is 550° C. or higher and 750° C. or lower.

After accelerated cooling has been performed, it is important to start reheating in a temperature range in which a non-transformed austenite phase is present. In this reheating process, when a reheating start temperature is equal to or lower than the Bf point, bainite transformation is completed, and a non-transformed austenite phase disappears. Therefore, it is necessary that the reheating start temperature be higher than the Bf point.

To ensure that C is concentrated in a non-transformed austenite phase, it is preferable that a steel plate be heated up to a temperature 50° C. or more higher than the reheating start temperature. It is not particularly necessary that a holding time be set during which the steel plate is held at the reheating temperature.

By using the manufacturing method, even when cooling is performed immediately after reheating has been performed, a sufficient amount of MA is achieved. As a result, a decrease in yield ratio and an increase in uniform elongation are achieved. However, to ensure that sufficient volume fraction of MA is achieved by promoting the diffusion of C into a non-transformed austenite phase more, the steel plate may be held at the reheating temperature for 30 minutes or less.

When the temperature holding time is more than 30 minutes, there may be a decrease in strength due to the occurrence of recovery of a bainite phase. In addition, it is fundamentally preferable to perform cooling using an air-cooling method after reheating has been performed.

As an apparatus to perform reheating after accelerated cooling has been performed, a heating apparatus may be equipped downstream of a cooling apparatus to perform accelerated cooling. Among heating apparatuses, it is preferable to use a gas-fired furnace or an induction heating apparatus with which a steel plate can be heated at a high heating rate.

As described above, first, rolling is performed under conditions such that the cumulative rolling reduction ratio is 50% or more in a temperature range of 900° C. or lower, which is a no-recrystallization temperature range in austenite. With this method, there is an increase in the number of MA formation sites as a result of a decrease in austenite grain size, and fine MA can be uniformly dispersed, which results in a low yield ratio of 85% or less in the state of a steel plate and of 90% or less in the state of a steel pipe being achieved.

Moreover, by performing reheating at a high heating rate after accelerated cooling has been performed, an increase in MA grain size is suppressed so that the equivalent circle diameter of MA is decreased to 3.0 μm or less. In addition, by starting cooling at a temperature equal to or higher than (the Ar₃ transformation temperature −60° C.) and equal to or lower than the Ar₃ transformation temperature, a polygonal ferrite phase is formed in the surface portions within 5 mm from the upper and lower surfaces, and, by performing cooling at a cooling rate of 200° C./sec. or less in terms of the surface temperature of a steel plate down to a temperature of 600° C. or lower in terms of the surface temperature of a steel plate, a bainite phase in the surface portion of the steel plate can be softened, which results in an elongation of 22% or more in the state of a steel plate and of 20% or more in the state of a steel pipe being achieved in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards.

With this method, even if a thermal history which causes deterioration of properties due to strain ageing in the case of a conventional steel is applied, MA is less likely to decompose in the steel, and it is possible to maintain the specified metallographic structure in which a duplex-phase structure consisting of a bainite phase and MA is mainly included and surface portions within 5 mm from the upper and lower surfaces have a three-phase structure consisting of a polygonal ferrite phase, a bainite phase, and MA.

As a result, even if a thermal history is applied at a temperature of 250° C. for 30 minutes, which is classified into a thermal history at a high temperature and for a long duration among common coating processes for steel pipes, an increase in yield stress (YS) that is caused by strain aging and accompanied by an increase in yield ratio and a decrease in uniform elongation can be suppressed. That is, the specified properties in the state of a steel plate and in the state of a steel pipe can be assured in the steel even if a thermal history which causes deterioration of properties due to strain ageing in a conventional steel plate is applied.

When a steel pipe is manufactured using the steel plate, the steel plate is formed into a cylindrical shape and the butted portions thereof are welded under conditions such that single-layer welding is performed on each of the outer and inner surfaces. Then, by performing a pipe-expanding treatment under a condition of a pipe expanding ratio of 0.4% or more and 2.0% or less, a steel pipe having good roundness can be obtained.

EXAMPLES 1

The steels (steel types A through J) having the chemical compositions given in Table 1 were cast into slabs using a continuous casting method, and thick steel plates (Nos. 1 through 17) having a thickness of 20 mm, 28 mm or 33 mm were manufactured.

TABLE 1 Steel Chemical Composition (mass %) Type C Si Mn P S Al Nb Ti Cu Ni Cr Mo A 0.032 0.20 2.5 0.008 0.001 0.03 0.034 0.014 — — — — B 0.051 0.56 1.8 0.008 0.002 0.04 0.023 0.011 0.24 0.20 — — C 0.072 0.06 1.8 0.011 0.001 0.03 0.044 0.013 — — — 0.22 D 0.064 0.15 1.7 0.008 0.001 0.03 0.021 0.009 0.20 0.20 — 0.18 E 0.054 0.15 2.2 0.008 0.001 0.04 0.025 0.008 — — 0.10 — F 0.058 0.16 1.7 0.009 0.001 0.03 0.009 0.016 0.16 0.18 0.03 0.20 G 0.063 0.13 1.9 0.008 0.001 0.03 0.014 0.013 — — — 0.20 H 0.023 0.38 2.4 0.008 0.002 0.03 0.032 0.010 — — — — I 0.062 0.65 1.1 0.009 0.001 0.03 0.024 0.011 — — — 0.10 J 0.071 0.34 2.2 0.008 0.001 0.03 0.035 0.014 — — — — Ar₃ Transformation Steel Chemical Composition (mass %) Point Type V Ca B N O (° C.) Ti/N Note A — — — 0.004 0.002 700 3.5 Example B — — — 0.005 0.001 734 2.2 Example C — — — 0.004 0.001 726 3.3 Example D — 0.0018 — 0.005 0.002 725 1.8 Example E — — — 0.005 0.002 716 1.6 Example F 0.030 0.0016 — 0.006 0.002 725 2.7 Example G — — 0.0010 0.004 0.002 722 3.3 Example H — — — 0.005 0.001 711 2.0 Comparative Example I — — 0.0008 0.004 0.002 795 2.8 Comparative Example J — — — 0.004 0.002 734 3.5 Comparative Example Annotation: Underlined value is out of our range. Annotation: Ar₃ Transformation Point (° C.) = 910-310C—80Mn—20Cu—15Cr—55Ni—80Mo (An atomic symbol represents the content (mass %) of the chemical element represented by the symbol.)

Immediately after hot-rolling had been performed on the heated slabs, cooling was performed using a water-cooling type accelerated cooling apparatus, and then reheating was performed using an induction heating furnace or a gas-fired furnace. The induction heating furnace was equipped on the same line as an accelerated cooling apparatus.

The manufacturing conditions of the steel plates (Nos. 1 through 17) are given in Table 2. Temperature such as a heating temperature, a rolling finish temperature, a cooling stop (finish) temperature, and a reheating temperature were represented in terms of the average temperature of the steel plate. The average temperature was calculated from the surface temperature of the slab or the steel plate using parameters such as thickness and thermal conductivity.

In addition, a cooling rate is an average cooling rate which was derived by dividing a temperature difference necessary in a cooling process after hot rolling has been finished down to a cooling stop (finish) temperature (430° C. to 630° C.) by a time spent for the cooling. In addition, reheating rate (heating rate) is an average heating rate which was derived by dividing a temperature difference necessary in a reheating process after cooling had been finished up to a reheating temperature (530° C. to 680° C.) by a time spent for the reheating.

TABLE 2 Cooling in Terms of Surface Temperature*1 Cooling Stop Temperature Cumulative Cooling of Steel Rolling Start Cooling Plate Reduction Temperature Rate of Surface Ratio Rolling of Steel Steel (Before Heating at 900° C. Finish Plate Plate Heat Steel Thickness Temperature or under Temperature Surface Surface Recovery) No. Type (mm) (° C.) (%) (° C.) (° C.) (° C./sec.) (° C.) 1 A 20 1130 65 760 680 130 390 2 B 20 1120 60 740 700 135 430 3 C 33 1080 70 750 710 120 410 4 D 20 1180 70 750 700 140 420 5 E 28 1050 60 740 690 135 340 6 F 33 1150 55 750 710 130 400 7 G 28 1150 75 770 720  95 370 8 E 20  970 75 750 690 135 410 9 E 20 1150 40 730 700 140 380 10 E 20 1180 75 760 680 130 400 11 F 28 1100 65 740 700 135 260 12 F 28 1200 60 790 690 135 410 13 F 28 1080 70 760 720 140 350 14 F 28 1080 70 760 720 230 200 15 H 20 1150 75 760 700 140 420 16 I 20 1090 70 820 740 140 310 17 J 28 1180 75 760 690 135 380 Cooling in Terms of Average Temperature*2 Average Average Cooling Cooling Stop Rate Temperature of Steel of Steel Reheating Reheating Plate Plate Reheating Rate Temperature No. (° C./sec.) (° C.) Apparatus (° C./sec.) (° C.) Note  1 30 590 Induction   1.2 650 Example Heating Furnace  2 35 630 Induction 3 650 Example Heating Furnace  3 20 610 Induction 3 680 Example Heating Furnace  4 40 620 Induction 5 650 Example Heating Furnace  5 35 540 Gas-fired 2 680 Example Furnace  6 30 600 Induction 3 660 Example Heating Furnace  7 35 570 Induction 5 650 Example Heating Furnace  8 35 610 Induction 7 680 Comparative Heating Example Furnace  9 40 580 Induction 5 650 Comparative Heating Example Furnace 10  3 600 Induction 6 680 Comparative Heating Example Furnace 11 35 430 Induction 5 650 Comparative Heating Example Furnace 12 35 610 Induction   0.2 680 Comparative Heating Example Furnace 13 40 550 Induction 7 530 Comparative Heating Example Furnace 14 40 550 Induction 7 650 Comparative Heating Example Furnace 15 40 620 Induction 6 650 Comparative Heating Example Furnace 16 40 510 Induction 7 680 Comparative Heating Example Furnace 17 35 580 Induction 2 650 Comparative Heating Example Furnace Annotation: Underlined value is out of our range. *1Cooling is controlled on the basis of a surface temperature of a steel plate. *2Cooling is controlled on the basis of an average temperature of a steel plate.

The mechanical properties of the steel plate manufactured as described above were determined. The results are given in Table 3. Tensile strength was evaluated on the basis of the average value thereof derived by collecting 2 test pieces for a full-thickness tensile test in a direction (C direction) at a right angle to the rolling direction and by performing a tensile test. The required strength is a tensile strength of 590 MPa or more. A yield ratio and an elongation were evaluated by collecting test pieces for a full-thickness tensile test in a direction (C direction) at a right angle to the rolling direction and by performing a tensile test. The required deformation performances are a yield ratio of 85% or less, a uniform elongation of 10% or more and a total elongation of 22% or more.

The toughness of a base metal was evaluated, by collecting 3 full-size V notch Charpy test pieces in a direction at a right angle to the rolling direction, by performing a Charpy test, by determining absorbed energy at a temperature of −40° C. and by calculating the average value of the 3 values. When the absorbed energy at a temperature of −40° C. was 200 J or more was evaluated as good.

Regarding toughness in a weld heat affected zone (HAZ), 3 test pieces that had been subjected to a thermal history corresponding to a heat input of 70 kJ/cm using a reproducing apparatus of weld thermal cycles were collected, and a Charpy impact test was performed for those test pieces. Then, absorbed energy at a temperature of −40° C. was determined, and the average value for the 3 test pieces was calculated. A case where the Charpy absorbed energy at a temperature of −40° C. was 100 J or more was evaluated as good.

TABLE 3 Polygonal Ferrite Area MA Fraction Equivalent Within C Direction in Plate State MA Circle 5 mm from (Before Ageing Treatment Fraction Diameter Upper and of 250° C.′ 30 min.) in Steel in Steel Lower Tensile Uniform Steel Thickness Plate Plate Surfaces Strength Yield Ratio Elongation No. Type (mm) (%) (mm) (%) (MPa) (%) (%) 1 A 20 11  1.6 60 621 75 12 2 B 20 8 1.2 50 610 74 12 3 C 33 13  2.6 40 677 71 11 4 D 20 7 1.7 50 622 75 13 5 E 28 6 1.6 55 655 73 11 6 F 33 10  1.3 70 662 78 13 7 G 28 4 1.5 40 636 70 12 8 E 20 1 2.5 50 556 89  9 9 E 20 7 3.5 55 608 77 12 10 E 20 2 2.4 40 620 87 11 11 F 28 0 1.5 60 655 94  9 12 F 28 1 1.6 35 660 83  8 13 F 28 0 1.3 40 633 89  9 14 F 28 5 1.8 50 645 84 12 15 H 20 1 1.4 50 655 88  8 16 I 20 0 1.8 35 581 86  9 17 J 28 14  4.3 40 643 66 12 C Direction in Plate State (Before Ageing Treatment of 250° C.′ 30 min.) Hardness Variation Hardness Base In Variation Metal HAZ Total Thickness in Width Surface Toughness Toughness Elongation Direction Direction Hardness vE-40° C. vE-40° C. No. (%) DHV DHV HV (J) (J) Note  1 24 20 25 222 307 141 Example  2 25 15 18 203 312 124 Example  3 23 24 23 214 294 118 Example  4 25 22 19 211 274 164 Example  5 26 14 16 220 318 155 Example  6 24 18 13 217 333 131 Example  7 25 11 14 216 361 182 Example  8 26 22 23 210 335 178 Comparative Example  9 27 25 24 222 129 124 Comparative Example 10 24 23 22 211 273 138 Comparative Example 11 21 26 34 232 285 161 Comparative Example 12 20 23 16 224 288 144 Comparative Example 13 20 25 25 215 312 116 Comparative Example 14 21 35 32 235 277 136 Comparative Example 15 25 16 18 207 293 122 Comparative Example 16 21 22 27 227 281 133 Comparative Example 17 21 34 33 233 302  28 Comparative Example Annotation: Underlined value is out of our range. Annotation: Tensile test was performed according to GOST (long gauge length).

In Table 3, in all our examples Nos. 1 through 7, the chemical compositions and the manufacturing conditions were in our range, and each of these examples had a high strength of 590 MPa or more in terms of tensile strength, a low yield ratio of 85% or less, a high uniform elongation of 10% or more, a high total elongation of 22% or more and good toughness for a base metal and a weld heat affected zone.

In addition, the microstructure in the central portion of the steel plate included mainly a bainite phase in which MA is dispersed, in which the area fraction of MA was 3% or more and 15% or less, and in which the equivalent circle diameter of MA was 3.0 μm or less. The area fraction of MA was determined by performing image processing on a microstructure observed using a scanning electron microscope (SEM). In addition, the microstructure in the surface proportions of the steel plate included mainly a polygonal ferrite phase and a bainite phase, in which MA is dispersed, and in which the area fraction of a polygonal ferrite phase was 10% or more and 80% or less.

On the other hand, in the comparative examples Nos. 8 through 14, the chemical compositions were in our range, but the manufacturing methods were out of our range. As a result, the microstructures were out of our range, and the yield ratio or the elongation was insufficient or the sufficient strength or toughness was not obtained in both conditions of before and after a strain ageing treatment at a temperature of 250° C. for 30 minutes. In Nos. 15 through 17, since the chemical compositions were out of our range, the yield ratio and uniform elongation of No. 15 was out of our range, and the tensile strength, yield ratio, uniform elongation and elongation of No. 16 were all out of our range. The toughness in a weld heat affected zone (HAZ) of No. 17 was out of our range.

Subsequently, UOE steel pipes were manufactured using the steel plates (Nos. 1 through 17) that had been manufactured under conditions given in Table 2.

After the obtained steel plates were formed by performing U-press and O-press, using a submerged arc welding method, inner surface seam welding was performed, and then outer surface seam welding was performed. Subsequently, by performing pipe-expanding treatment under a condition of a pipe-expanding ratio of 0.6% to 1.2%, steel pipes having an outer diameter of 400 to 1626 mm were manufactured. Using a tensile test piece in accordance with GOST standards cut out of the base metal of the steel pipe, tensile properties were evaluated. In addition, using a tensile test piece in accordance with GOST standards cut out of a sample material by the same method described above, which had been cut out of the base metal of the pipe to form the tensile test pieces and subjected to an ageing treatment at a temperature of 250° C. for 30 minutes, tensile properties after having undergone ageing treatment were evaluated. In addition, using a V-notch Charpy impact test piece according to JIS Z 2202 (1980) cut out of the central portion in the thickness direction of the base metal of the steel pipe, a Charpy impact test was performed at a test temperature of −40° C. Moreover, using a test piece for a DWTT (Drop Weight Tear Test) according to API-5L cut out of the steel pipe, a DWTT was performed at a test temperature of −20° C. to determine an SA value (Shear Area: ductile fracture area ratio). In addition, using a V-notch Charpy impact test piece according to JIS Z 2202 (1980) cut out of the portion of the outer surface FL (Fusion Line) of the welded joint of the steel pipe, a Charpy impact test was performed at a test temperature of −40° C. The notch was formed at a position where the HAZ and the weld metal were included at a ratio of 1:1.

The test results are given in Table 4.

TABLE 4 Polygonal C Direction in UOE Pipe State MA Ferrite (Before Ageing Treatment of 250° C.′ 30 min.) Equivalent Area Fraction Base MA Circle within 5 mm Metal Fraction Diameter from Upper Toughness HAZ in Steel in Steel and Lower Yield Tensile Yield Uniform Total DWTT Toughness Steel ThickNess Plate Plate Surfaces Strength Strength Ratio Elongation Elongation vE-40° C. SA-20° C. vE-40° C. No. Type (mm) (%) (mm) (%) (MPa) (MPa) (%) (%) (%) (J) (%) (J) 1 A 20 11  1.6 60 532 626 85 10  22 307 100 141 2 B 20 8 1.2 50 517 616 84 10  23 312 100 124 3 C 33 13  2.6 40 547 675 81 9 21 294 100 118 4 D 20 7 1.7 50 534 628 85 11  23 274 100 164 5 E 28 6 1.6 55 547 659 83 9 24 318 100 155 6 F 33 10  1.3 70 559 665 84 12  22 333  95 131 7 G 28 4 1.5 40 514 642 80 10  23 361 100 182 8 E 20 1 2.5 50 514 559 92 7 24 335  95 178 9 E 20 7 3.5 55 509 613 83 10  25 129  75 124 10 E 20 2 2.4 40 571 628 91 9 22 273  95 138 11 F 28 0 1.5 60 599 663 90 7 19 285 100 161 12 F 28 1 1.6 35 559 666 84 6 18 288 100 144 13 F 28 0 1.3 40 581 639 91 7 18 312 100 116 14 F 28 5 1.8 50 554 652 85 11  19 277 100 136 15 H 20 1 1.4 50 599 661 91 6 23 293  95 122 16 I 20 0 1.8 35 534 580 92 7 19 281  95 133 17 J 28 14  4.3 40 518 648 80 10  19 302  60  28 C Direction in Ageing Treated Pipe State (After Ageing Treatment of 250° C.′ 30 min.) Base Uni- Metal L Direction in Ageing Treated Pipe State form Total Toughness HAZ (After Ageing Treatment of 250° C.′ 30 min.) Yield Tensile Yield Elonga- Elonga- DWTT Toughness Yield Tensile Yield Uniform Total Strength Strength Ratio tion tion vE-40° C. SA-20° C. vE-40° C. Strength Strength Ratio Elongation Elongation No. (MPa) (MPa) (%) (%) (%) (J) (%) (J) (MPa) (MPa) (%) (%) (%) Note 1 544 632 86 9 22 321 100 132 542 630 86 10  22 Example 2 531 625 85 10  22 304 100 133 530 623 85 11  21 3 575 684 84 10  21 288 100 122 571 680 84 10  22 4 532 633 84 11  23 268 100 141 529 630 84 10  23 5 557 671 83 9 23 307 100 146 549 661 83 9 24 6 584 679 86 10  21 311  95 120 580 674 86 11  22 7 534 651 82 10  22 341 100 152 533 650 82 11  22 8 524 570 92 7 23 311  95 141 521 566 92 7 22 Com- 9 543 617 88 10  24 134  75 102 540 614 88 9 24 parative 10 538 626 86 8 21 266  95 108 536 623  6 8 22 Example 11 620 674 92 5 19 277 100 114 617 671 92 6 20 12 569 677 84 6 17 269 100 138 567 675 84 7 18 13 588 646 91 6 18 274 100 104 586 644 91 7 19 14 572 657 87 8 18 266 100 122 570 655  7 8 18 15 608 675 90 7 22 288  95 133 606 673 90 7 23 16 535 588 91 7 18 278  95 103 533 586 91 7 18 17 544 664 82 9 19 245  60  19 542 661 82 9 19 Annotation: Underlined value is out of our range. Annotation: Tensile test was performed according to GOST (long gauge length).

Our target ranges regarding the base metal of a pipe are: a tensile strength of 590 MPa or more, an elongation of 20% or more, and a ratio of a 0.5%-proof stress to a tensile strength of 90% or less, where all of those were determined before and after an ageing treatment at a temperature of 250° C. for 30 minutes. At the same time, our target ranges regarding the base metal are a Charpy absorbed energy at a test temperature of −40° C. of 200 J or more and a DWTTSA −20° C. (ductile fracture area ratio in a DWTT test at a test temperature of −20° C.) of 85% or more, and our target range regarding the seam weld joint of steel pipe is a Charpy absorbed energy of 100 J or more at an outer surface FL notch at a temperature of −40° C.

In Table 4, in all our examples Nos. 1 through 7, the chemical compositions and the manufacturing methods were in our ranges. Therefore, these examples had a high tensile strength of 590 MPa or more, a low yield ratio of 90% or less and a high elongation of 20% or more before and after a strain ageing treatment at a temperature of 250° C. for 30 minutes, and, further, the toughness of a base metal and in a weld heat affected zone were good.

On the other hand, in the comparative examples Nos. 8 through 14, the chemical compositions were in our range, but the manufacturing methods were out of our range. As a result, the microstructures were out of our range, and the yield ratio or elongation was insufficient or the sufficient strength or toughness was not obtained in both conditions of before and after a strain ageing treatment at a temperature of 250° C. for 30 minutes. In Nos. 15 through 17, since the chemical compositions were out of our range, the yield ratio and uniform elongation of No. 15 were out of our range, and the tensile strength, yield ratio, uniform elongation and elongation of No. 16 were all out of our range. The elongation and toughness in a weld heat affected zone of No. 17 were out of our range. 

The invention claimed is:
 1. A high strength steel plate having a low yield ratio, the steel plate having a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure including a three-phase structure of mainly a polygonal ferrite phase and a bainite phase in which island martensite is dispersed in surface portions within 5 mm from the upper and lower surfaces, and a duplex-phase structure of mainly a bainite phase in which island martensite is dispersed in a central portion which is other than the surface portions, wherein an area fraction of island martensite is 3% or more and 15% or less, wherein an equivalent circle diameter of the island martensite is 3.0 μm or less, wherein the area fraction of the polygonal ferrite phase in the surface portions is 10% or more and less than 80%, and wherein the remainder of the metallographic structure consists of a bainite phase, a hardness variation in a thickness direction of ΔHV30 or less in terms of Vickers hardness, a hardness variation in a width direction of ΔHV30 or less in terms of Vickers hardness, a maximum hardness in surface portions of the steel plate of HV230 or less in terms of Vickers hardness and a yield ratio of 85% or less and an elongation of 22% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards.
 2. The steel plate according to claim 1, wherein the chemical composition further comprises, by mass %, one or more selected from among Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Ca: 0.0005% or more and 0.003% or less and B: 0.005% or less.
 3. A method of manufacturing a high strength steel plate having a low yield ratio comprising: heating steel having the chemical composition according to claim 1 at a temperature of 1000° C. or higher and 1300° C. or lower, performing hot rolling under conditions such that cumulative rolling reduction ratio is 50% or more in a temperature range of 900° C. or lower, finishing hot rolling at a temperature equal to or higher than the Ar₃ transformation point, starting cooling when the temperature of the surface portions within 5 mm from the upper and lower surfaces is equal to or higher than (the Ar₃ transformation temperature −60° C.) and equal to or lower than the Ar₃ transformation point, performing the cooling at a cooling rate of 200° C./sec. or less in terms of a surface temperature of the steel plate until the surface temperature becomes 600° C. or lower, performing the cooling at a cooling rate of 15° C./sec. or more in terms of the average temperature of the steel plate until the average temperature becomes 450° C. or higher and 650° C. or lower, and, immediately after the cooling has been performed, performing reheating at a heating rate of 1.0° C./sec. or more in terms of the surface temperature of the steel plate until the surface temperature becomes 550° C. or higher and 750° C. or lower.
 4. A high strength welded steel pipe, the steel pipe being manufactured by forming the steel plate according to claim 1 into a cylindrical shape, by welding butted portions of the shaped cylinder under conditions such that single-layer welding is performed on each of the outer and inner surfaces, and by thereafter performing a pipe-expanding treatment under a condition of a pipe expanding ratio of 0.4% or more and 2.0% or less, and having a yield ratio of 90% or less and an elongation of 20% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards and, further having a yield ratio of 90% or less and an elongation of 20% or more even after a strain ageing treatment has been performed under conditions such that the temperature is 250° C. or lower and the duration is 30 minutes or less.
 5. A method of manufacturing a high strength steel plate having a low yield ratio comprising: heating steel having the chemical composition according to claim 2 at a temperature of 1000° C. or higher and 1300° C. or lower, performing hot rolling under conditions such that the cumulative rolling reduction ratio is 50% or more in a temperature range of 900° C. or lower, finishing hot rolling at a temperature equal to or higher than the Ar₃ transformation point, starting cooling when the temperature of the surface portions within 5 mm from the upper and lower surfaces is equal to or higher than (the Ar₃ transformation temperature—60° C.) and equal to or lower than the Ar₃ transformation point, performing the cooling at a cooling rate of 200° C./sec. or less in terms of a surface temperature of the steel plate until the surface temperature becomes 600° C. or lower, performing the cooling at a cooling rate of 15° C./sec. or more in terms of the average temperature of the steel plate until the average temperature becomes 450° C. or higher and 650° C. or lower, and, immediately after the cooling has been performed, performing reheating at a heating rate of 1.0° C./sec. or more in terms of the surface temperature of the steel plate until the surface temperature becomes 550° C. or higher and 750° C. or lower.
 6. A high strength welded steel pipe, the steel pipe being manufactured by forming the steel plate according to claim 2 into a cylindrical shape, by welding butted portions of the shaped cylinder under conditions such that single-layer welding is performed on each of the outer and inner surfaces, and by thereafter performing a pipe-expanding treatment under a condition of a pipe expanding ratio of 0.4% or more and 2.0% or less, and having a yield ratio of 90% or less and an elongation of 20% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards, and, further having a yield ratio of 90% or less and an elongation of 20% or more even after a strain ageing treatment has been performed under conditions such that the temperature is 250° C. or lower and the duration is 30 minutes or less. 